Naozumi Sudou is a high school student and a member of the student council at the school attended by the protagonist Shiina Tamai. On the surface, he appears calm, polite, and well-liked by both peers and teachers. However, this demeanor masks a deeply sociopathic nature. He is willing to commit serious crimes such as grand theft auto and murder without hesitation, all in service of his personal ideology. He claims to have surpassed ordinary human emotional discomfort and desire, and he views himself as a protector of the Earth. In practice, this means he believes that humanity is wasting the planet's limited resources and that only those who have a meaningful impact on the world deserve to live. He seeks to dismantle tradition, custom, and common sense, replacing them with a society structured purely around individual ability.

Naozumi is driven by a profound misanthropy and a will to nothingness, openly stating that he represents the power of destruction and that his ultimate aim is to extinguish. He is both pessimistic and cynical, yet he maintains a mild and unassuming manner in everyday interactions, making his true intentions difficult to detect early on. As the leader of the Black Children's Association, also called the Star Chamber Club, he unites other owners of dragon children under his philosophical banner. He orchestrates the kidnapping of the character Akira Sakura by sending Satomi Ozawa to determine whether Akira possesses a dragon, believing that controlling such beings is key to reshaping the world.

In the story, Naozumi initially appears as a supportive friend to Shiina, encouraging her to join the student council. As events unfold, he reveals himself to be manipulative and controlling, using others as pawns for his own agenda. His key relationships include his association with Komori, as well as his calculated interactions with Shiina and Akira. He does not form genuine bonds but rather leverages people to further his plans. Over the course of the narrative, his cold and dangerous nature becomes increasingly apparent.

Notably, Naozumi is bonded with a dragon child named Trickster. Trickster primarily serves as an observer and possesses abilities such as teleportation and object duplication. While these powers could be used offensively, Naozumi employs them efficiently for surveillance and strategic purposes rather than direct combat, reflecting his analytical and strategic mindset. His character arc exemplifies the series' exploration of how absolute power warps individuals with strong but twisted convictions.